The subject matter disclosed herein relates to a metrology device, such as a portable articulated arm coordinate measurement machine (AACMM) or laser tracker, and to a metrology device that can compensate for synchronous and asynchronous bearing runout error.
A metrology device is a device that may be used by an operator to measure and electronically capture coordinates of points on a surface. There are several different types of metrology devices, such as but not limited to: an AACMM, a laser line probe, a laser tracker and an optical structured light scanner for example. The coordinates of the measured points are determined relative to a local coordinate reference system of the measurement device. For example, the AACMM may have a local coordinate system positioned in the base of the device. The local coordinate reference system allows each of the measurements made by the measurement device to be taken relative to each other. The coordinate reference system may be made during post processing.
Typically, the metrology device, such as an AACMM for example, will have a number of rotational joints. The rotation of these joints is monitored using a sensor such as a rotary encoder. One source of error in the measurement of the coordinates of the measured points is due to bearing runout error at each rotary joint. Bearing runout error has two components: 1) a synchronous error that repeats each rotation; and 2) an asynchronous error that changes with each rotation. It should be appreciated that while the synchronous error may be compensated in the calculation of the coordinates, asynchronous error is more difficult as the coordinate measurement device needs to track the number of rotations of the bearing assembly. The tracking of the bearing rotation is difficult since the rotary joint may be moved when the coordinate measurement device is in an unpowered state.
Accordingly, while existing metrology devices are suitable for their intended purposes the need for improvement remains, particularly in providing a metrology device that compensates for synchronous and asynchronous errors in rotary joints.